Fixed the audio backgrounding bug

content/browser/renderer_host/media/audio_renderer_host.cc

Index: content/browser/renderer_host/media/audio_renderer_host.cc
diff --git a/content/browser/renderer_host/media/audio_renderer_host.cc b/content/browser/renderer_host/media/audio_renderer_host.cc
index df16bce5d9655443698009f1fc8820c89bc1a94f..8555aab6ada708b02cbccd2d85d83c5eb49fbf07 100644
--- a/content/browser/renderer_host/media/audio_renderer_host.cc
+++ b/content/browser/renderer_host/media/audio_renderer_host.cc
@@ -104,6 +104,16 @@ std::string TranslateDefaultId(const std::string& device_id) {
                            : device_id;
 }
 
+void NotifyRenderProcessHostThatAudioStateChanged(int render_process_id) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  RenderProcessHost* render_process_host =
+      RenderProcessHost::FromID(render_process_id);
+
+  if (render_process_host)
+    render_process_host->AudioStateChanged();
+}
+
 }  // namespace
 
 class AudioRendererHost::AudioEntry
@@ -756,11 +766,27 @@ void AudioRendererHost::UpdateNumPlayingStreams(AudioEntry* entry,
         !RenderFrameHasActiveAudio(entry->render_frame_id());
     entry->set_playing(true);
     base::AtomicRefCountInc(&num_playing_streams_);
+
+    // Inform the RenderProcessHost when audio starts playing for the first
+    // time.
+    if (base::AtomicRefCountIsOne(&num_playing_streams_)) {
+      BrowserThread::PostTask(
+          BrowserThread::UI, FROM_HERE,
+          base::Bind(&NotifyRenderProcessHostThatAudioStateChanged,
+                     render_process_id_));
+    }
   } else {
     entry->set_playing(false);
     should_alert_resource_scheduler =
         !RenderFrameHasActiveAudio(entry->render_frame_id());
-    base::AtomicRefCountDec(&num_playing_streams_);
+
+    // Inform the RenderProcessHost when there is no more audio playing.
+    if (!base::AtomicRefCountDec(&num_playing_streams_)) {
+      BrowserThread::PostTask(
+          BrowserThread::UI, FROM_HERE,
+          base::Bind(&NotifyRenderProcessHostThatAudioStateChanged,
+                     render_process_id_));
+    }
   }
 
   if (should_alert_resource_scheduler && ResourceDispatcherHostImpl::Get()) {